Mobile systems generally refer to different telecommunication systems that enable private wireless data transmission for subscribers moving within the system. A typical mobile system is a public land mobile network (PLMN). The PLMN comprises fixed radio stations (base stations) located in the service area of the mobile network, the radio coverage areas (cells) of the base stations providing a uniform cellular network. A base station provides in the cell a radio interface (air interface) for communication between a mobile station and the PLMN. Since mobile stations can move in the network and they have access to the PLMN through any base station, the PLMNs are provided with complicated arrangements for subscriber data management, authentication and location management of mobile subscribers, for handovers (a change of a base station during a call) etc. The networks are also provided with services that support the transmission of information other than the usual speech calls (speech service), such as data, facsimile, video image, etc. These new services have required a considerable amount of developmental work and new arrangements in the networks.
Another area of mobile systems includes satellite-based mobile services. In a satellite system, radio coverage is obtained with satellites instead of terrestrial base stations. The satellites are located on an orbit circling the earth and transmitting radio signals between mobile stations (or user terminals UT) and land earth stations (LES). The beam of the satellite provides on the earth a coverage area, i.e. a cell. The coverage areas of individual satellites are arranged to form continuous coverage so that a mobile station is located at all times within the coverage area of at least one satellite. The number of the satellites needed depends on the desired coverage. Continuous coverage on the surface of the earth might require for example 10 satellites.
Subscriber mobility requires similar arrangements in satellite mobile systems as in the PLMNs, i.e. subscriber data management, authentication and location management of mobile subscribers, handovers, etc. The satellite systems should also support similar services as the PLMNs.
One way of implementing these requirements in satellite mobile systems is to use existing PLMN arrangements. In principle this alternative is very simple since a satellite system can be basically compared to a base station system of a mobile system having an incompatible radio interface. In other words, it is possible to use a conventional PLMN infrastructure where the base station system is a satellite-system. In such a case, the same network infrastructure could in principle even contain both conventional PLMN base station systems and satellite "base station systems".
There are many practical problems related to the adaptation of the PLMN infrastructure and a satellite system, however. A problem apparent to the Applicant is that a PLMN traffic channel and a traffic channel of a "radio interface" in a satellite system differ considerably. Examine an example where the PLMN is the Pan-European digital mobile system GSM (Global System for Mobile Communication) and the satellite mobile system is the Inmarsat-P system that is currently under development.
A traffic channel in the GSM system supports data transmission at the user rates of 2400, 4800, 7200 and 9600 bit/s. In the future, high-speed data services (HSCSD=High speed circuit switched data) employing two or more traffic channels at the radio interface (multi-slot access) also support higher user rates (14400 bit/s, 19600 bit/s, . . . ). A data connection provided by one traffic channel is V.110-rate-adapted. A V.110 connection is a digital transmission channel that was originally developed for ISDN (Integrated Services Digital Network) technology and that is adapted to a V.24 interface. In V.110 frames, terminal interface status information (V.24 interface control signals), such as CT105 (RTS=ready to send), CT108 (DTR=data terminal ready), CT106 (CTS=clear to send), CT107 (DSR=data set ready) and CT109 (CD=Data carrier detect), is also transmitted in both transmission directions in addition to the user data. Further, in multichannel transparent HSCSD data service it is also necessary to transfer intersubchannel synchronization information. The aforementioned additional information increases the bit rate at the radio interface higher than the actual user rate. The radio interface rates corresponding to the user rates of 2400, 4800 and 9600 bit/s are 3600, 6000 and 12000 bit/s. In addition, the traffic channel employs channel coding that aims at decreasing the effect of transmission errors.
The Inmarsat-P satellite system requires that standard data rates up to 4800 bit/s can be supported by one traffic channel (e.g. 1200, 2400, 4800 bit/s) and that standard data rates exceeding 4800 bit/s (e.g. 9600, 14400, 19200 bit/s, etc.) can be supported by using several parallel traffic channels, such as in the HSCSD service of the GSM system.
In the Inmarsat-P satellite system, a data rate of one traffic channel at the radio interface is at most 4800 bit/s, which equals the user data rate of 4800 bit/s at the terminal interface. In a data service employing two traffic channels the data rate at the radio interfaces equals the user data rate of 9600 bit/s at the terminal interface. A problem occurs when not only the user data but also the above-described terminal interface status information and possible intersubchannel synchronization information should be transmitted over the radio interface. Therefore the protocol data unit, i.e. the frame structure, used by the satellite system at the radio interface should be defined to carry the aforementioned control and synchronization information over the radio interface. One manner would be to use directly the GSM system arrangement, i.e. a V.110-based frame structure, also at the radio interface of the satellite system. However, this would be a very complicated arrangement and it would significantly reduce the user data rates available. A single traffic channel could not support the user data rate of 4800 bit/s since the V.110 frame structure and the terminal interface status information increase the actual data rate higher than 4800 bit/s. Therefore the highest standard user data rate on one traffic channel would be 2400 bit/s. For the same reason, a two traffic channel data service could not support the user rate of 9600 bit/s, but the highest standard user data rate would be 4800 bit/s (or in some systems 7200 bit/s). A corresponding decrease in the available data rates would also occur in data services employing more than two traffic channels. Such an arrangement where the overhead information causes a significant loss of capacity would not be satisfactory.
A similar problem can also occur when connecting other types of radio interfaces, such as wireless telephone systems, to PLMNs.